Effects of rainfall and land use on nutrient responses in rivers in the Brazilian semiarid region.

This paper investigated whether rainfall promotes dilution or increase in nutrient concentrations and which land use indicators are the main predictors of nutrients in intermittent rivers in a large Brazilian semiarid region. The total phosphorus (TP) and total inorganic nitrogen (TIN) were monitored between 2013 and 2018 at 92 river water quality monitoring sites. The monthly rainfall (Rn) was obtained from 575 rain gauges. Pearson's correlation (R) between Rn and nutrient concentration was performed. The correlation patterns were also analysed based on land use data: urban area (%), agricultural field area (%), demographic density (inhabitants/km2), sewer system coverage (%), and reservoir density (reservoir/km2). Backward stepwise regression was performed to identify predictors of nutrient concentrations. The results revealed a marginal effect of rainfall on nutrients when the effects of urbanisation outweigh all other aspects. However, in regions with greater accumulated rainfall and lower reservoir density, the rainfall was related to a linear increase in nutrient concentrations (R > 0.8). Contrastingly, in the basins with less accumulated rainfall and greater inter-basin hydrological disconnection, there was a linear reduction in nutrient concentration (R < - 0.5). In the backward stepwise regression, sewer system coverage and Rn had the greatest influence for TP, and the urban area was the strongest predictor for TIN. Importantly, our results demonstrated that in semiarid rivers in densely populated regions, there is no single pattern of variability in nutrient concentration, on a wide scale of assessment. Therefore, adaptative and decentralised management can be more effective in improving water quality in these regions.

Modelling the impact of sediment management on the trophic state of a tropical reservoir with high water storage variations.

Eutrophication of lakes has affected society in many regions, particularly in water scarce environments where: i) low runoff reduces the self-purification potential of water bodies; ii) water supply relies on surface reservoirs, which are susceptible to nutrient enrichment. This work presents an assessment of the impact of the silted sediment management on the trophic status of a tropical surface reservoir with intense temporal variability of water storage. A complete mixing model describing the total phosphorus budget in the water and sediments was used, based on semi-empirical formulations. The sediment reuse as soil fertilizer has been proposed to increase productivity in small scale agriculture, which should also enhance the water quality by removing the nutrient-enriched sediment from lakes. Model application for a 40-years period indicate that sediment management may improve water quality, changing from poor to acceptable trophic state during roughly 10% of the time when the reservoir is not empty.